It's impossible to do it on O3DS because of the way Extended Memory games work. The Memory occupied by everthing which is loaded on homemenu (so e.g. NTR, too) is unloaded at the Moment the game boots. This is the reason why the 3ds has to be restarted after game Exit.
The games wouldn't run on O3DS otherwise. O3DS has not enough RAM to do this.
Even if you would find an exploit in the game you want to cheat. You would have to load the Code first and even this would be a very hard Task to do without crashing the game.
So either get yourself a n3ds or modify the game itself and run it.